A medical implant or device must satisfy a number of requirements. Factors affecting the choice of the medical implant or device and the material thereof are mainly all mechanical properties and biocompatibility. The material must not cause any inflammatory reaction or allergic reaction. Commonly used materials often include nickel, like medical grade 316L stainless steel, which contains about 16% nickel. For patients with an allergic reaction the implantation of such materials is contraindicated. Another consideration in material selection is the need for the implanting physician to be able to visualize the position of the medical implant or device during procedure to the desired target site in the body, and for purposes of examination from time to time thereafter at the implant site, typically by X-ray fluoroscopy.
With the growing importance of magnetic resonance imaging (MRI), MRI compatibility is desirable. The metal alloys commonly used for implantation (like stainless steel 316) induce a local disturbance of the magnetic field used in MRI, to the extent that imaging of surrounding tissue is impeded. Although alloys like Nitinol behave more favourably in MRI, their MRI compatibility is not considered to be sufficiently good.
This invention relates to medical devices or implants in general such as catheters, guide wires, stents, stent grafts and heart valve repair devices.
Stents are generally thin walled tubular-shaped devices composed of complex patterns of inter-connecting struts which function to hold open a segment of a blood vessel or other body lumen like oesophagus and urethra. Stent grafts are stents with a circumferential covering or lining and are suitable for supporting a dissected artery or intimal flap that can occlude a vessel lumen. Stents and stent grafts are typically implanted by use of a catheter. Initially they are maintained in a radially compressed state to manoeuvre them through the lumen. Once in position, they are deployed. The material from which the vascular prosthesis like stents or stent grafts is constructed must allow the prosthesis to undergo expansion, which typically requires substantial deformation. Once expanded the stent must maintain its size and shape and must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel lumen. The wall of the prosthesis must be sufficiently thick, depending on the stent material, not only to withstand the vessel wall recoil but also allow the stent to be seen on the fluoroscope. Finally, the prosthesis material must be biocompatible so as not to trigger any adverse vascular responses like restenosis or thrombus formation in the treated vessel.
For medical devices such as all kind of catheters and guide wires special mechanical properties are desired to have perfect trackability and pushability during the intervention. Moreover, good radio-opacity and MRI compatibility are essential in order to survey medical procedures via x-ray and MRI. Finally also for these medical devices biocompatibility is a must.
In the past years increased effort was undertaken to find new materials for medical implants and devices bearing superior characteristics over commonly used metals like stainless steel or titanium. Numerous publications focus on titanium alloys aiming at corrosion resistant, high strength and biocompatible alloys. As described for example in U.S. Pat. No. 6,312,455, US 2001/0007953, and WO 99/58184 many Titanium-alloys thereof are super-elastic or shape memory alloys. A pseudo-elastic β-titanium alloy fabricated from Titanium, Molybdenum, Aluminium and optionally Niobium, Chrome and Vanadium is described in U.S. Pat. No. 6,258,182. EP 0 788 802 provides a self-expanding stent consisting of a titanium alloy including at least about 68 weight percent titanium and optionally Niobium, Zirconium, and Molybdenum. U.S. Pat. No. 6,238,491 and WO 00/68448 describe a Niobium-Titanium-Zirconium-Molybdenum alloy for medical devices providing a uniform β-structure, which is corrosion resistant, and can be processed to develop high-strength and low-modulus. The alloy comprises 29 to 70 weight percent Niobium, 10 to 46 weight percent Zirconium, 3 to 15 weight percent Molybdenum and a balance of Titanium. In another approach Davidson (EP 0 601 804) employ an alloy consisting essentially of Titanium, 10 to 20 or 25 to 50 weight percent Niobium and optionally up to 20 weight percent Zirconium, the alloy having an elastic modulus less than 90 GPa. Similar Titanium-alloys for medical implants also published by Davidson comprise Titanium, 10 to 20 or 35 to 50 weight percent Niobium and optionally up to 20 weight percent each Zirconium and Tantalum (EP 0 437 079) or Titanium, 10 to 20 or 35 to 50 weight percent each Niobium and Tantalum and optionally up to 20 weight percent Zirconium (U.S. Pat. No. 5,690,670). EP 0 707 085 also provides a low modulus, biocompatible Titanium-base alloy for medical devices consisting of 20 to 40 weight percent Niobium, 4.5 to 25 weight percent Tantalum, 2.5 to 13 weight percent Zirconium and the balance Titanium. A further high strength, low modulus and biocompatible Titanium-alloy is laid open in U.S. Pat. No. 4,857,269 and EP 0 359 446 consisting of Titanium and up to 25 weight percent Niobium, Zirconium, and Molybdenum. EP 1 046 722 describes a corrosion resistant Titanium-Zirconium-type alloy for medical appliances consisting of 25 to 50 weight percent Titanium, 5 to 30 weight percent Niobium, 5 to 40 weight percent Tantalum and 25 to 60 weight percent Zirconium.
Further approaches to develop biocompatible, high strength alloys which are also sufficiently radio-opaque and do not contain Titanium are described in U.S. Pat. No. 6,478,815 and WO 02/43787. Both documents reveal stents made from at least 90 weight percent Niobium. Niobium is a relatively soft and ductile metal, which is alloyed with traces of other elements, e.g. Zirconium, Tantalum or Titanium for reinforcement of the alloy. However, Niobium surfaces cannot be electropolished because of their tendency to smear. Stents fabricated from binary Tantalum-Alloys, namely Tantalum-Niobium and Tantalum-Tungsten, are disclosed in WO 02/05863.